The present invention generally pertains to electronic article surveillance (EAS) systems and is particularly directed to an improvement in a presence detection system used for detecting the presence of an article within a surveillance zone.
An EAS presence detection system includes means for transmitting electromagnetic radiation of a first predetermined frequency into a surveillance zone; a tag for attachment to an article to be detected within the surveillance zone, with the tag containing a transponder that responds to detection of electromagnetic radiation of the said first predetermined frequency by transmitting electromagnetic radiation of a second predetermined frequency; and means for detecting radiation of the second predetermined frequency within the surveillance zone. The transponder includes one of a number of different types of materials that, when within a predetermined magnetic field intensity range, responds to detection of electromagnetic radiation of the said first predetermined frequency by transmitting electromagnetic radiation of a second predetermined frequency that is either a multiple harmonic or a frequency-divided quotient of the first predetermined frequency or at the first frequency. In the prior art, a magnetic bias field within said predetermined magnetic field intensity range is provided within the surveillance zone either by including within the transponder a bias strip of magnetic material that provides a magnetic bias field within said predetermined magnetic field intensity range or by disposing a permanent magnet within or adjacent the surveillance zone. Although the latter technique enables smaller and less expensive tags to be used, since the transponder need not include a bias strip, the latter technique may be less reliable because, due to normal processing variations during preparation of the transponder material that responds, not all transponders respond within the same predetermined magnetic field intensity range, and also because a magnetic bias field provided from a discrete source decreases in intensity with distance from the source. Also, ambient magnetic fields, which combine with the magnetic field provided by the magnetic field source to provide the resultant o magnetic bias field within the surveillance zone, vary from location to location and sometimes even within the surveillance zone due to variations in the Earth's magnetic field and the presence of different ferromagnetic structures near the surveillance zone at different locations. Even when the former technique is used, a non-uniform ambient magnetic field throughout the surveillance zone may prevent a transponder including a bias strip from responding when within a portion of a surveillance zone if the ambient magnetic field in such portion of the surveillance zone is of such intensity that the total magnetic field intensity in such portion resulting from a combination of the ambient magnetic field intensity and the magnetic field intensity provided by the bias strip included in the transponder is outside of the predetermined magnetic field intensity range.